The invention relates to a method for the field-oriented control of an induction motor by means of a frequency converter, in which method a transformation angle is determined by estimation and is corrected in dependence on a rotational speed of a rotor flux vector or of the induction motor and/or in dependence on a delay time.
For the field-oriented control of induction motors, it is normally necessary to have information about the magnitude and position of the rotor flux vector that is as accurate as possible. In an ordinary squirrel-cage motor, however, it is not normally possible to measure the rotor current. For that reason the rotor flux is estimated using an observer. The estimation comprises in this case a calculation using measured values and/or assumed values.
Different types of observers are known: W. Leonhard "Control of Electrical Drives" (Heidelberg, 1990, p. 214 ff.), and D. S. Wijesundera and R. D. Jackson "Observers for field-oriented control of induction motor drives" (IEE Proceedings-B, Vol. 139, 1992, p. 381 ff.) describe the construction of rotor flux vector observers, but these are relatively bulky and complicated.
An "observer" is normally a form of estimating device, which is also known as an estimator and which uses both the output data of a model and a feed-back loop to correct errors and improve the accuracy of the estimation. Such estimators are called "closed-loop observers", that is, estimators having a closed-loop control circuit. In addition to those there are "open-loop observers" without a feed-back loop, which generally carry out a real time simulation of the control, which results in very short response times. Moreover, because there is no feed-back loop the stability is often better. Opportunity for correction, however, is limited.
With such types of observer, both the current equation and the voltage equation can be used for field-oriented motor control. Lower-order models are adequate for that purpose. If it is desirable to evaluate both the current equation and the voltage equation of the induction motor, higher-order observers are required, which are accordingly complex and generally also require higher calculating capacity.
In observers which are based on the voltage equation, the stator voltage is measured. If one does not wish or is not able to measure the stator voltage, simpler direct feedback observers which are based on the current equation are used instead. The efficiency of these observers is relatively poor, however, because the estimation of the transformation angle is not accurate.
On the other hand, these simple observers with the correspondingly simpler estimating devices have the advantage of a simpler construction and shorter calculation times.